High temperature ruined her date (3,4)
I believe the answer is:
red heat
'high temperature' is the definition.
(red heat is a kind of high temperature)
'ruined her date' is the wordplay.
'ruined' indicates anagramming the letters.
'her'+'date'='herdate'
'herdate' is an anagram of 'RED HEAT'.
(Other definitions for red heat that I've seen before include "Fieriness" , "High temperature" , "Temperature at which something glows" , "Glowing temperature" .)